1. Field of the Invention:
This invention relates to injection molding apparatus. More specifically, this invention relates to mixing heads for combining separate fluid streams prior to entry into a mold cavity.
2. Description of the Prior Art:
In order to form molded plastic parts, a heated plastic material is injected into a mold under pressure where it cools and solidifies to form the molded part or parts. Reaction injection molding (RIM) has become increasingly popular as a low-cost alternative to conventional injection molding processes. This is due to the shorter cycle times inherent in RIM processes.
In reaction injection molding, two or more components are admixed immediately prior to entry into the mold. The components react as they mix to generate a reaction product which gels to conform to the confines of the mold. The reaction product can be a rigid or resilient polymeric material depending upon the formulation used. In RIM processes, one common method of producing the polymeric reaction product is by metering a liquid stream of a polyol pre-mix and an isocyanate stream into a mix head and delivering the combined material into the mold.
The mix head is typically a housing containing a piston movably mounted within a chamber or bore formed in the housing. A rod is mounted on the end of the piston. The rod extends through a second bore formed in the housing. Two inlets and an outlet are formed in the housing in communication with the second bore. Each inlet is connected to a pressurized liquid reactant stream while the outlet is connected to the mold.
The rod selectively opens and closes the inlets to the second bore in the housing. Retraction of the rod permits the liquid reactant material to flow from the inlets through the bore. The reverse movement of the rod closes the bore from the inlets and forces the polyol-isocyanate mixture from the bore into the mold where the polyol-isocyanate mixture reacts to form the desired molded part.
The quality of the resulting polymeric part depends, in large part, on the intimate mixture of the reactants. Inadequate mixing can result in non-uniform gelling characteristics which can weaken the resulting polymer. At the very least, inadequate mixing slows the reaction rate due to inadequate contact between the components.
To ensure proper mixing, aftermixers are sometimes added to the mixing heads of injection molding machines. These devices are placed in or adjacent to the bore or mixing chamber to mix the combined ingredients. One drawback of aftermixers is the tendency of the reacting material to gel and foul the device. This necessitates frequent cleaning and maintenance to ensure maximum efficiency.
Thus, it would be desirable to provide a means for mixing various reactants which does not require a mechanical aftermixer. It is also desirable that the means for mixing be of a type which is resistant to fouling and does not interfere with injection of the polymeric mixture into the mold.